Multi-winding brushless motors are commonly used as the drives of servo systems in a wide range of industrial applications from robotics and automation to aerospace and military. Accurate and ripple-free torque control of brushless motors is essential for precision control of such servo systems, and to avoid vibration in other applications.
In multi-winding brushless motors, electric power is distributed by an electronic controller (electronically controlled commutation system) to a plurality of windings, each delivering power to the motor during a respective phase (range of angular positions), of the motor. Conventional electronic controllers incorporate feedback from the rotor angular position. These are typically analog feedback circuits, although digital controllers are increasingly being used for flexibility and simplified control depending on operating requirements.
The basic function of the controller is to independently excite the windings of the motor to rotate the magnetic field generated by the windings, to rotate the rotor. Thus the controller is coupled by respective drive circuits to the windings. While most multi-winding brushless motors incorporate the windings in the stator, it is possible, though usually inconvenient, to locate windings in the rotor, and its logically possible to include windings on both the stator and rotor. The multi-winding brushless motors may divided into AC and DC motors, where the power input is characterized, or by a mechanism for imparting motion given the magneto-motive field (e.g. a permanent magnet, a reluctance-based material, inductive coil). Electrically commutated, multi-winding, motors, such as brushless DC motors, including permanent magnet synchronous motors, switched reluctance motors, and induction motors, require such a controller.
Conventional controllers excite respective windings with approximately sinusoidal current waveforms for smooth motor operation. However, non-ideal motors do not generate a perfectly sinusoidally distributed magneto-motive force from a sinusoidal current waveform excitation, and so a sinusoidal excitation applied to a non-idea motor can result in torque ripple. Torque ripple, a well known problem in the art, is a cyclic variation in the torque output of a motor.
It is well known that suppressing the torque ripple of the motor drive of a servo system can significantly improve system performance by reducing speed fluctuations[1,2]. Commercial high-performance electric motors reduce the pulsating torque by increasing a number of motor poles. However, such motors tend to be expensive, heavy, and bulky due to construction and assembly of multiple windings, rotor poles, the attendant drive circuits, etc.
Control approaches for accurate torque production in electric motors and their underlying models have been studied by several researchers [1-14]. It was assumed in these works that the currents applied to the drive circuits (i.e. drive currents) can be controlled accurately and instantaneously, so the currents were treated as the control inputs. Then, the waveforms of the drive currents were pre-shaped so that the generated torque is equal to the requested torque. However, when the drive circuits have current and voltage limits, some of them may not be able to deliver the drive currents dictated by the controller, for example, when the motor operates at high torque or speed, or when a winding fails. Consequently, the motor's torque production may significantly deteriorate as a result of the distortions of the drive current caused by the voltage or current saturation of the amplifiers.
Flux weakening is a known technique that allows a machine to operate above the rated speed in a constant-torque high-speed region, when there is a fixed inverter voltage and current [15]. Below the rated speed, all of the drive currents can be used to produce torque. Above the rated speed, a part of the drive current must be used to oppose the permanent magnet flux while the remaining portion is used to produce torque. Several authors have addressed flux weakening in permanent magnet synchronous motors [16-19]. However, these techniques can deal with electric motors with prefect sinusoidal back-emf waveforms, and current limits are not taken into account.
Accordingly there is a need for a technique for controlling drive circuits of multi-winding brushless motors that improves operation in the event of a failure mode, and for multi-winding brushless motors with improved accuracy of torque output.